1. Field of the Invention
This invention relates generally to a multi-mode sensor system located in a common transmitting/receiving aperture and, more particularly, to a tri-mode, co-boresighted sensor system located on an airborne platform, such as a missile seeker.
2. Description of Related Art
Single mode sensors used, for example, in missile seekers are well known in the state of the art but typically exhibit degraded performance because of false target acquisitions. In order to overcome this inherent deficiency, a dual-mode seeker including millimeter wave (MMW) and infrared (IR) sensors in a common aperture have been developed. One such system is shown and described in U.S. Pat. No. 5,214,438, entitled “Millimeter Wave and Infrared Sensor in a Common Receiving Aperture”, issued to T. C. Brusgard et al. on May 25, 1993. More recently, a tri-mode seeker additionally including a laser spot tracker has been developed by the assignee of the present invention and is shown and described in U.S. Pat. No. 6,606,066, entitled, “Tri-Mode Seeker” issued to J. M. Fawcett et al. on Aug. 12, 2003, the details of which are incorporated herein by reference.
In the Fawcett et al patent, the RF transmitter/receiver is located at the focus of a primary reflector located on a gimbal assembly. A selectively coated dichroic mirror is located in the path of the millimeter wave energy so as to reflect infrared energy from the primary reflector to an optical system which re-images the infrared energy on an infrared detector. The outer edge or rim of the primary reflector is additionally deformed so that incoming laser energy focuses to a location beyond the RF transmitter/receiver. A laser sensor is positioned adjacently behind the RF transmitter/receiver in a back-to-back orientation. The laser energy is then detected using a secondary reflector and an optical system which directs the laser energy from the secondary reflector to a laser detector. In such a configuration, the reception of laser energy is restricted to a relatively small zone on the outer periphery of the primary mirror, thus restricting the collecting aperture since it severely limits the amount of laser energy which can be detected. Also, the packaging is awkward and crowded, severely reducing the overall packaging efficiency.
Additionally, propagating a laser wavelength to the IR focal plane has also been attempted, but it degrades IR performance due to the limited selection of materials that pass all desired wavelengths and their color properties which make it impossible to fully color correct the optical design, particularly over the IR band. The constraints on material selections also raise an issue of electromagnetic interference (EMI) susceptibility in the IR detector apparatus.
Another attempt in the development of a tri-mode seeker placed the laser sensor at an intermediate image location, i.e., between the secondary mirror and the relay optics cell. While this offers a significant advantage to the IR path since the color correction and EMI issues are removed, there are other significant limitations which remain. These include distortion of the IR wave front and loss of image quality and a lack of volume for packaging the necessary support electronics. Also a narrow band filter is required for the laser sensor so that it can reject solar background. This location makes coating design very difficult, if not impossible, by demanding the coating also pass the IR band while imposing a wide range of incident angles that it must accommodate.
Thus, all prior approaches have inherent limitations which impose some form of penalty and/or difficulty in a suitable overall system design.